Prior art spectrophotometer systems frequently make use of a deuterium lamp as a light source. The advantage of such a lamp is that it burns nicely in a continuous manner, and it has a couple of well defined wave length peaks. However, the intensity of a deuterium lamp is relatively low. In a liquid chromatographic system the small flow cells requires high intensity light in order to make adequate detection possible. Also the optical system comprising e.g. fibre optics require much more light than is delivered by a deuterium lamp. Wavelength calibration in such systems is performed by locating a single peak. This inevitably yields problems of accuracy at the short wave and long wave ends respectively of the wave length scale. It is also very important that a selected wave length be reproducible. The reproducibility reachable in calibration should be at least .+-.2 nm.
U.S. Pat. No. 5,212,537 is directed to a method of calibration for monochromators and spectrophotometers, using a continuous light source, a tungsten filament mercury vapour lamp, having two sharp intensity peaks. The location of the peaks is identified by measuring the width of said peaks at half height, and the midpoint of this width is taken as the location of the peak. This technique would not be possible to use in connection with a flash lamp, e.g. a Xenon lamp, since the intensity of the peaks fluctuates.
U.S. Pat. No. 5,268,737 is directed to calibration of a spectrophotometer. As the source of continuous light, a deuterium lamp and a tungsten lamp is used. A point of reference is determined by using zero order light. This would not be possible in connection with the use of optical fibres for transmission.